Acoustic wave devices employing a piezoelectric thin film resonator are used as filters and duplexers of wireless devices such as mobile phones. The piezoelectric thin film resonator has a structure designed to have a lower electrode and an upper electrode facing each other across a piezoelectric film. A region where the lower electrode and the upper electrode face each other across the piezoelectric film is a resonance region.
Rapid diffusion of wireless system leads to usage of many frequency bands. This requires the filter and the duplexer to have steep skirt characteristics. Increasing the Q-value of the piezoelectric thin film resonator is one way to achieve steep skirt characteristics.
Japanese Patent Application Publication No. 2014-161001 (Patent Document 1) discloses inserting an insertion film in the piezoelectric film in the outer peripheral region of the resonance region.
The piezoelectric thin film resonator disclosed in Patent Document 1 can reduce the leakage of acoustic wave energy from the resonance region, enabling to improve the Q-value. However, the insertion film makes the upper surface of the piezoelectric film uneven in Patent Document 1. This deteriorates the characteristics such as the Q-value.